The present invention is in the field of catalysis. More particularly, the present invention is directed to supported precious metal catalysts, preferably palladium and/or gold metal catalysts, having a high degree of dispersion and a high degree of edge-coating. The present invention is further directed to a process for producing these catalysts, as well as to the use of these catalysts in chemical reactions.
Many supported precious metal (PM) catalysts are used inter alia for selective conversion of fine chemicals, which are typically highly functionalized large molecules. In order for the PM crystallites to be best accessible for the large molecules, it is desirable to have them situated in the outer shell of the support as much as possible. This is referred to as an edge-coated metal distribution. Typically a high degree of edge-coating is achieved when the majority of the PM crystallites are situated in the outer shell of the support. Such an edge-coated distribution has the additional advantage that the substrate molecule rapidly leaves the vicinity of the PM crystallite after it is converted, thereby minimizing any side reactions from taking place. However, the preparation of edge-coated catalysts with a high dispersion is difficult. Since only a small portion of the total surface area of the support is used, edge-coated catalysts generally have a low dispersion.
Supported precious metal catalysts are typically prepared by a deposition-reduction method, in which the metal is first deposited on a support and then reduced. In order to prepare a catalyst with a high dispersion, typically a deposition-precipitation preparation procedure is carried out in which the metal salt has time to diffuse into the pores of the support. In this way, a high dispersion is achieved by the metal being distributed over the complete surface area of the support. This provides a large active metal surface area since the formed metal crystallites are very small, typically in the order of a few nanometers. On the other hand, for the preparation of edge-coated catalysts, the metal salt should not diffuse too deep into the pores of the support (i.e. typically less than 100 nm into the pores for a support particle having an average size of about 20 to 100 micron), but should deposit immediately upon contact with the support. These two methods contradict each other, so the preparation of high-dispersed edge-coated metal catalysts is typically a compromise between these two factors.
Heterogeneous catalysts based on colloidal suspensions are characterized by small metal crystallite sizes, see for instance P. T. Witte, S. Boland, F. Kirby, R. van Maanen, B. F. Bleeker, D. A. M. de Winter, J. A. Post, J. W. Geus, P. H. Berben, Chem Cat Chem. 5 (2013) 582-587. The present invention aims at producing catalysts having even smaller precious metal particle (crystallite) sizes, in particular nanop articles that are about the same size and/or smaller than the average pore size of the support. Furthermore, the present invention aims at producing catalysts wherein the supports are edge coated with the PM crystallites.
Also WO-A-2009/096783 describes the preparation of an aqueous colloidal precious metal suspension, which process comprises reducing a precious metal salt in aqueous solution using a functionalized, water soluble quaternary ammonium salt in the absence of organic solvents, to form elementary nanoparticles. Although this known method yields small precious metal particles, there is still a need for catalysts having even smaller precious metal crystallites, which are distributed as described above.
In U.S. Pat. No. 4,212,824 a platinum on a carbon black support catalyst is described with an improved metal distribution as determined by ESCA (electron spectroscopy for chemical analysis), but in this document the metal dispersion is not specified. Also, the carbon black support used in this document has a low surface area (i.e. 20-100 m2/g).
Accordingly it is a first object of the present invention to provide a precious metal catalyst, in particular a palladium or gold metal catalyst, which is not only edge coated but also has a high metal dispersion. Another object of the present invention is to provide a process for producing said catalyst. A further object of the present invention is to provide said catalyst, so that it may be used in chemical reactions.